1. Field of Invention
The present invention relates generally to improving the tenderness of meat, and, more specifically, to methods for improving meat tenderness through the use of hypercalcemic agents to activate or enhance postmortem tenderization mechanisms.
2. Background
Recent surveys have indicated that an excessive amount of variation exists in the tenderness of beef cuts at the retail and food service levels (Morgan et al., 1991; Hamby, 1992). In fact, the latest National Beef Quality Audit ranked inadequate tenderness as the second most important beef quality problem (Smith et al., 1995). It is estimated that the annual economic loss associated with beef toughness equals $7.64 per animal or $217.0 million to the U.S. beef industry. Thus, the National Cattlemen's Beef Association (NCBA, 1995) listed development of a procedure or instrument that can improve and/or predict beef tenderness as a high-priority issue within the U.S. beef industry.
In the past, attempts at tenderizing meat have involved extended holding times or aging of meat postmortem in a controlled, refrigerated environment. Davey and Gilbert (1969) first provided evidence that associated the involvement of calcium ions with the process of postmortem aging of meat. Busch et al. (1972) later demonstrated that the presence of calcium induced myofibril fragmentation and that process in turn was hindered by the addition of a calcium chelator. Many subsequent efforts have focused on the relationship between postmortem storage and the activities of the calpain protease system (i.e., .mu.- and m-calpain and their endogenous inhibitor, calpastatin) (for review see Koohmaraie, 1992). Because the activity of m-calpain remains nearly constant throughout postmortem storage, and there is a progressive decrease in the activity of .mu.-calpain, Koohmaraie et al. (1987) suggested that .mu.-calpain, not m-calpain, was involved in postmortem aging. However, when sufficient calcium ions are present, both calpain proteases are activated and each undergoes autolysis. Therefore, the rapid loss of .mu.-calpain during postmortem storage (Koohmaraie et al., 1987) is a good indicator that .mu.-calpain, unlike m-calpain, is activated during normal postmortem conditions. Koohmaraie et al. (1989) validated this theory when ovine carcasses infused with calcium chloride solution produced meat which was tender after only 1 day of postmortem aging. These results led to the development of the calcium-activated tenderization (CAT) system in which prerigor or postrigor meat is injected with a calcium chloride solution, which in turn activates the both calpain proteases and results in enhanced meat tenderness (Wheeler et al. 1991, 1992, 1993, 1994, and 1997).
Currently, there is a debate on how calpain proteases could possibly function in muscle tissue since the calcium requirements for proteolytic activity of these enzymes (approximately 10 .mu.M for .mu.-calpain and approximately 200-300 .mu.M for m-calpains) are much higher than the free calcium concentrations found in living muscle tissue (&lt;1 .mu.M). While the concentration of free calcium in resting muscle is low, postmortem muscle calcium concentrations can reach 100 .mu.M as a result of the release of calcium ions from mitochondrial and sarcoplasmic reticulum reserves. Previous studies (Koohmaraie et al., 1987; Vidalenc et al., 1983; Ducastaing et al., 1985) have indicated that under normal postmortem conditions, m-calpain is remarkably stable, whereas there is a gradual decline in the activities of .mu.-calpain, and calpastatin loses its activity rapidly. Both .mu.- and m-calpain undergo autolysis in the presence of sufficient calcium with the eventual loss of activity (Suziki et al., 1981; DeMartino et al., 1986; Koohmaraie et al., 1989). Secondly, elevation of calcium, which results in accelerated postmortem proteolysis and tenderization process, also accelerates the process of calpastatin inactivation (Koohmaraie et al., 1988; 1989).
Attempts have been made to elevate muscle calcium concentrations through dietary calcium supplementation and/or infusion of a calcium chloride solution to a live animal, but since blood calcium homeostasis is regulated very closely (8-12 mg/dL in cattle) these attempts have resulted in limited success. Accordingly, prior methods proposed for improving tenderness generally have involved mechanical, chemical or enzyme application to the meat tissue. Such systems are costly and raise consumer concerns about food adulteration and allergic reactions. Pre-harvest infusions of calcium mixtures directly into the blood stream immediately pre-slaughter will improve meat tenderness, but such administration is complex to control.
It is accordingly an object of the present invention to provide a method for increasing the tenderness of livestock meat tissues that is easy to accomplish, commercially practicable, and which does not raise consumer concerns.